The most important rule is to always leave campsites looking better than when you found them. Make it look like people hadn't been there for a long time.
These are all part of the basic Leave No Trace principles. The best tip I can give is for everyone to familiarize themselves with the seven LNT principles, follow them yourself, and pass them on to others.
Plan Ahead and Prepare, Travel and Camp on Durable Surfaces, Dispose of Waste Properly, Leave What You Find, Minimize Campfire Impact, Respect Wildlife, and Be Considerate of Other Visitors
These should all result in a continuous and safe environments for future hikers/campers. Outside of the designated camp spots, nobody should even know you were there. Hence, the "leave no trace"
We actually do positive impact camping by collecting any rubbish we find on the way. We just have to be careful as there is the risk of needles, spiders, and other nasties when collecting rubbish.